In oil and gas exploration and production, engineers require geological and petrophysical data on the hydrocarbon formation within a reservoir in order to evauluate the oil/gas yield and to determine the optimum drilling and extraction program. A technique commonly used to obtain petrophysical data is core sampling. It is the only method of making direct measurement of rock and fluid properties.
In this approach a well is drilled and at predetermined depths a core sample is taken. A core sampling tool is attached to the end of the drill string. The tool includes a core barrel on which is located a core bit being a cylindrical blade with teeth mounted on the forward circular end. As the drill string is rotated the teeth cut through the rock formation and a solid cylindrical rock sample is obtained. As the cutting occurs the sample enters the core barrel and passes into an inner tube or liner which carries the sample to the surface.
On the surface, the liner is extracted from the core barrel and divided into smaller sections for transportation to the laboratory. Known disadvantages of this technique is that the core sample can be damaged due to movement of the sample within the liner during transportation; the liner can flex causing unwanted fractures in the core sample; and soft friable sediments within the core sample may lose adhesion from the core and fall away, making sections of the core unsuitable for analysis.
In an attempt to overcome these disadvantages, various stabilizing techniques have been proposed to hold the core sample intact within the liner. In one technique, liquids such as resins or plasters (gypsum) have been injected into the annulus between the sample and the inner wall of the liner. Once set, the core sample is then prevented from moving in relation to the liner during transportation. However, this technique has a number of inherent disadvantages. As the core sample comprises a rock matrix including fractures and pores, the liquid mixtures enter these areas, forcing out at least some the hydrocarbon fluid content and water as it seeps through the sample. Thus the resin/plaster invades the pores. The injection pressure can also cause disruption and destruction of the rock formation rendering useless much analysis data collected in the laboratory. Yet further as these liquids work by gravitational drainage, they can only flow where there is a totally open annulus. As a result they have limited success where the core sample contains friable sediments.
An alternative technique for stabilizing core samples is freezing. This can be done in a freezer, using dry ice or dipping a core in liquid nitrogen. Besides the inherent difficulty in transporting the material and equipment to undertake freezing on a rig, the frozen sample must remain frozen, as any thawing will damage the core. Freezing cannot be used for samples from gas reservoirs and the method and local conditions are critical to the analysis of the core in the laboratory. If the core is frozen slowly, damage to grain boundaries results and measurements of resistivity, sonic velocity and permeability are affected. Additionally, there will be marked fluid migration which influences saturation determination and prevents chemical tracers being used on the core sample. Freezing at a faster rate to overcome the disadvantages of grain boundary damage and increased fluid migration, however, causes fracturing along thin bed boundaries due to the large thermal shocks experienced.